According to our current understanding, the issue is devolved.  The law in Scotland is the Indecent Displays (Control) Act 1981, and the English equivalent is the Obscene Publications Act.

Prosecutions and convictions under these acts are rare.  Neither 'indecent' or 'obscene' is defined, and due to this fact it seems that only the most extreme porgnography is likely to result in a conviction.

The acts state that indecent or obscene material should effectively be stocked in a separate part of the shop behind an adequate notice prohibiting under 18s.

In spite of the fact that these laws have clearly been created to protect under 18s from being exposed to inappropriate material, the material in question is based on a definition of what adults find "indecent" or "obscene", rather than a definition of what is and is not deemed suitable for children's eyes.